Polar group-containing thermoplastic resins such as saponified ethylene-vinyl ester-based copolymer (hereinafter, sometimes referred to as “EVOH-based resin”), polyvinyl alcohol-based resin (hereinafter, sometimes referred to as “PVA-based resin”), polyamide-based resin (hereinafter, sometimes referred to as “PA-based resin”) and the like are excellent in gas-barrier property and therefore have been utilizing for generally forming into a container or wrapping film for foods by melt-molding.
Since the polar group-containing thermoplastic resin is easy to adhere to the wall of melt flow passageway, in the case of switching from the polar group-containing thermoplastic resin to a new resin as a molding material to be fed into a molding apparatus, the molding apparatus is needed to be clean. Before switching to the new resin, the preceding resin (the polar group-containing thermoplastic resin) which is remaining in the melt flow passageway or adhering to the melt flow passageway should be removed by the use of purging agent.
Also in the case that a polar group-containing thermoplastic resin is continued to be fed to a molding apparatus such as extruder for a long time, a part of the polar group-containing thermoplastic resin stays for a long hours in the passageway of the apparatus. Such stagnation of the resin results in gelation, thermal deterioration or decomposition of the resin, which causes contamination of the resulting molded article. Thus obtained molded article is a defective one with streak or gel, which is a problem.
If such a defective product with streak or gel is extruded from the extruder, the extrusion operation needs to be stopped. In this situation, purging the apparatus of the resin remaining in the passageway or adhering to the wall of the passageway is required as an effective solution of the problem.
As an appropriate purging agent which is effective for removing polar group-containing thermoplastic resin such as EVOH-based resin, there is a known purging agent comprising a hydrocarbon-based resin composition containing a relatively large amount of metal salt such as inorganic salt or organic salt as suggested in, for example, patent document 1.
The document 1 explains that the purging agent can make an excellent cleaning effect due to the fact that the metal salt is transferred from the purging agent to the resin to be removed and brings the resin to decomposition.
Also there is a known purging agent for processing hydrophilic resin such as EVOH-based resin as disclosed in, for example, patent document 2. The purging agent comprises a hydrous resin obtained by adding an appropriate amount of water to thermoplastic resin as a main component.
This purging agent can give water to the hydrophilic resin, thereby the resin is softened and improved in its flowability, and as a result, an excellent cleaning effect may be obtained.